Base station apparatus for controlling downlink transmission power of a small-cell base station

ABSTRACT

A base station apparatus of small-cell base station is provided, by which interference from a small-cell in downlink signals of macro cell can be suppressed without lowering a power of downlink signals from the small-cell base station more than necessary, in state that a macro-cell base station is located neighboring the small cell of small-cell base station. The base station apparatus comprises transmission-power control means of controlling a down link transmission power of downlink in a range between a maximum power and a minimum power which are set in advance, and information acquisition means of acquiring information on electrical field intensity of a transmission signal transmitted from the macro-cell base station located at a periphery of the own base station. The transmission-power control means changes the downlink transmission power so as to change the downlink transmission power to the minimum power or stop the transmission of the downlink transmission signal, when the electric field intensity of transmission signal from the macro-cell base station exceeds a preset electric-field upper threshold, or the electric field intensity is equal to or larger than the electric-field upper threshold, in a state that the downlink transmission power is set to the maximum power.

TECHNICAL FIELD

The present invention relates to a base station apparatus of mobilecommunication.

BACKGROUND ART

A base station apparatus of performing a handover (HO) processing when amobile station (hereinafter referred to appropriately as “user equipment(UE: User Equipment)”) located in the own cell moves to a neighboringcell of another base station while maintaining a data communication,etc. in a mobile communication is conventionally known. When performingthis HO processing, since information on the neighboring base stations(cells) is needed in advance, it is necessary that a neighboring-celllist that is a list of cell-identification information of theneighboring base stations (cells) is created in each base stationapparatus (for example, refer to Non-Patent Literature 1). For example,as a neighboring-cell list referred to when a mobile station located ina cell of a base station (eNB: evolved Node B), to which a physical cellidentifier (PCI: Physical Cell Identifier) of 500 is assigned, performsa handover in a mobile communication system of LTE (Long Term Evolution)system, a neighboring-cell list exemplified in Table 1 is created. Inthe neighboring-cell list in Table 1, a cell ID (Cell ID),identification information on a communications service provider (PLMN(Public Land Mobile Network) ID) and a location-registration area code(TAC: Tracking Area Code) are stored together with the physical cellidentifier (PCI) with respect to each neighboring base station (cell).

TABLE 1 PCI Cell ID PLMN ID TAC 499 XXX YYY ZZZ AAA BBB CCC DDD . . . .. . . . . . . .

As a method of creating the foregoing neighboring-cell list, a method ofsearching peripheral base stations, which is called as “Sniffer”, and amethod using receiving reports, which is called as “CGI Report”, “UEMR”,etc. are known. In the foregoing method of searching peripheral basestations, information on peripheral base stations (cells) is searchedwith a particular frequency (for example, 2.1 GHz) at a timing ofactivating a base station or in a predetermined cycle (for example, onehour or one week) that is set in advance, and a new peripheral basestation is registered in the neighboring-cell list when the peripheralbase station is searched. In the foregoing method using receivingreports, while not being restricted by a particular frequency, a basestation collects information on a peripheral base station based onglobal identification information (CGI: Cell Global Identity), which isincluded in a measurement report (MR) received from a mobile stationlocated in the own cell when performing a handover to the peripheralbase station, and registers the information on peripheral base stationin the neighboring-cell list. Since the information to be registered inthe neighboring-cell list is collected by a plurality of methods in thisway, there is a case that information on a method (information source)of acquiring information on the neighboring base station (cell) isincluded in the neighboring-cell list together with the information onthe neighboring base station (cell). For example, as shown in Table 2,in a neighboring-cell list of a small-cell base station locatedneighboring with base stations A, B and B, information on an informationsource (Source) of acquiring cell ID information is stored together withthe cell ID.

TABLE 2 Source Cell ID Sniffer A Sniffer B CGI Report C

A base station of small cell smaller than a macro-cell in size(hereinafter referred to appropriately as “small-cell base station”) isalso known. A base station of macro-cell (hereinafter referred toappropriately as “macro-cell base station”) is intentionally located. Onthe other had, the small-cell base station is not intentionally located,and is located, for example, at a position where a radio wave strengthis weak.

A base station apparatus of the foregoing small-cell base station canperform a down link power control (DPC: Downlink Power Control) bymeasuring an interference level (CRS_Ec) of a downlink specificreference signal (CRS: Cell-specific Reference Signal) and a totaldownlink receiving power from neighboring cells by a peripherallistening function (for example, refer to Non-Patent Literature 2). Inthe conventional algorithm of DPC, a control target value of downlinktransmission power to be used after that is determined based onmeasurement results of the foregoing measured instantaneous interferencelevel and downlink receiving power.

CITATION LIST Non-Patent Literature

-   Non-Patent Literature 1: 3GPP TS36.331 V10.3.0, “Evolved Universal    Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC);    Protocol specification”, published on Oct. 10, 2011.-   Non-Patent Literature 2: 3GPP TS36.104 6.2.5-1.-   Non-Patent Literature 3: 3GPP TS 36.331, “Evolved Universal    Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC);    Protocol specification”.

SUMMARY OF INVENTION Technical Problem

There is a problem that, in state that the macro-cell base station islocated neighboring with the small-cell base station, interferencebetween the small cell and the macro cell easily interfere with eachother and interference from the small cell to the macro cell increases.

Solution to Problem

To solve the foregoing problem, a base station apparatus according to anaspect of the present invention, which is a base station apparatusinstalled in a small-cell base station performing a radio communicationwith a mobile station in a mobile communication system, comprisestransmission-power control means of controlling a downlink transmissionpower in a range between a maximum power and a minimum power which areset in advance, and information acquisition means of acquiringinformation on electrical field intensity of a transmission signaltransmitted from the macro-cell base station located at a periphery ofthe own base station. The transmission-power control means changes thedownlink transmission power so as to change the downlink transmissionpower to the minimum power or stop the transmission of the downlinktransmission signal, when the electric field intensity of transmissionsignal from the macro-cell base station exceeds a preset electric-fieldupper threshold, or the electric field intensity is equal to or largerthan the electric-field upper threshold, in a state that the downlinktransmission power is set to the maximum power.

In the foregoing base station apparatus, the transmission-power controlmeans may change the downlink transmission power so as to return thedownlink transmission power to the maximum power, when the electricfield intensity of downlink signal from the macro-cell base station issmaller than a preset electric-field lower threshold, or the electricfield intensity is equal to or smaller than the electric-field lowerthreshold, in a state that the downlink transmission power is set to theminimum power or the transmission of downlink transmission signal isstopped. Herein, the electric-field upper threshold and theelectric-field lower threshold may be different from each other or sameas each other.

Furthermore, in the foregoing base station apparatus, thetransmission-power control means may perform the change of downlinktransmission power after confirming that a mobile station is not presentin the cell of own base station.

Moreover, in the foregoing base station apparatus, thetransmission-power control means may control so that the maximum poweris maintained while not performing the change of downlink transmissionpower to change the downlink transmission power to the minimum power orstop the transmission of downlink transmission signal, even when theelectric field intensity of transmission signal from the macro-cell basestation exceeds the electric-field upper threshold, or the electricfield intensity is equal to or larger than the electric-field upperthreshold, in state that a function of suppressing interference betweenthe small cell of own base station and a peripheral base station isoperated.

Further, in the foregoing base station apparatus, the number of times ofdetermining whether the electric field intensity of transmission signalfrom the macro-cell base station exceeds the electric-field upperthreshold, or whether the electric field intensity is equal to or largerthan the electric-field upper threshold within a predetermined time isdefined as “N times”, and the number of times of the macro-cell electricfield exceeding the electric-field upper threshold or being equal to orlarger than the electric-field upper threshold among the determinationsof N times is defined as “Count”, and the change of downlinktransmission power may be performed to change the downlink transmissionpower to the minimum power or stop the transmission of downlinktransmission signal when the value of Count/N exceeds a preset thresholdor the value of Count/N is equal to or larger than the threshold.

Furthermore, in the foregoing base station apparatus, the number oftimes of determining whether the electric field intensity oftransmission signal from the macro-cell base station is smaller than theelectric-field lower threshold, or whether the electric field intensityis equal to or smaller than the electric-field lower threshold within apredetermined time is defined as “N times”, and the number of times ofthe macro-cell electric field being smaller than the electric-fieldlower threshold or being equal to or smaller than the electric-fieldlower threshold among the determinations of N times is defined as“Count”, and the change of downlink transmission power may be performedto return the downlink transmission power to the maximum power when thevalue of Count/N exceeds a preset threshold or the value of Count/N isequal to or larger than the threshold.

Advantageous Effects of Invention

According to the present invention, in state that a macro-cell basestation is located neighboring a small cell of small-cell base station,interference from the small-cell in the downlink signals of macro cellcan be suppressed without lowering a power of downlink signals from thesmall-cell base station more than necessary.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration showing a schematic configuration of mobilecommunication system in which a base station having a base stationapparatus is located, according to an embodiment of the presentinvention.

FIG. 2A is a function block diagram showing one example of schematicconfiguration of a main part of user equipment capable of communicatingin the mobile communication system according to the present embodiment.

FIG. 2B is a function block diagram showing one example of schematicconfiguration of a main part of a base station apparatus forming asmall-cell base station according to the present embodiment.

FIG. 3 is a flowchart showing one example of update processing of aneighboring-cell list in the base station apparatus of the small-cellbase station according to the present embodiment.

FIG. 4 is a flowchart showing another example of update processing of aneighboring-cell list in the base station apparatus of the small-cellbase station according to the present embodiment.

FIG. 5A is a graph showing examples of algorithm of downlinktransmission power controls (DPCs) different from each other, which arecapable of selectively performing when interference from a small cell toa macro cell is large, in the small-cell base station according to thepresent embodiment.

FIG. 5B is a graph showing examples of algorithm of downlinktransmission power controls (DPCs) different from each other, which arecapable of selectively performing when interference from a small cell toa macro cell is large, in the small-cell base station according to thepresent embodiment.

FIG. 5C is a graph showing examples of algorithm of downlinktransmission power controls (DPCs) different from each other, which arecapable of selectively performing when interference from a small cell toa macro cell is large, in the small-cell base station according to thepresent embodiment.

FIG. 6 is a flowchart showing one example of a downlink transmissionpower control (DPC) in the base station apparatus of small-cell basestation according to the present embodiment.

FIG. 7 is a flowchart showing another example of a downlink transmissionpower control (DPC) in the base station apparatus of small-cell basestation according to the present embodiment.

FIG. 8 is a graph showing yet another example of algorithm of downlinktransmission power control (DPC) in the base station apparatus ofsmall-cell base station according to the present embodiment.

FIG. 9A is a graph showing a control pattern of power of downlinktransmission signals with respect to a frequency at the point A in FIG.8.

FIG. 9B is a graph showing a control pattern of power of downlinktransmission signals with respect to a frequency at the point B in FIG.8.

FIG. 9C is a graph showing a control pattern of power of downlinktransmission signals with respect to a frequency at the point C in FIG.8.

FIG. 10 is a flowchart showing yet another example of a downlinktransmission power control (DPC) in the base station apparatus ofsmall-cell base station according to the present embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention are described withreference to the drawings.

FIG. 1 is an illustration showing a schematic configuration of mobilecommunication system in which a base station having a base stationapparatus is located, according to an embodiment of the presentinvention. In FIG. 1, the communication system of this embodiment is acommunication system based on the specification of LTE, and includesmacro-cell base stations 10, 11 and 12, a small-cell base station 20located in a macro cell 10A being as a radio communication area of themacro-cell base station 10 which is a one of the foregoing macro-cellbase stations. A small cell 20A being as a radio communication area ofthe small-cell base station 20 is included within the macro cell 10A. Inthe example shown in the figure, user equipment (UE) 30 being as amobile station is located in the small cell 20A and is in a statecapable of performing radio communications for a telephone and datacommunication etc. to the small-cell base station. Since the userequipment 30 is also located in an outer peripheral portion (boundaryportion with the macro cell 10A) of the small cell 20A within the macrocell 10A, the user equipment 30 is in a situation in which a radiosignal transmitted from the user equipment 30 reaches to the macro-cellbase station 10 and a radio signal transmitted from the macro-cell basestation 10 reaches to the user equipment 30. As a base station locatedaround the small-cell base station 20, there are macro-cell basestations 11 and 12 in addition to the macro-cell base station 10.

It is noted that, although three macro-cell base stations 11, 12 and 13,one small-cell base station 20 and one user equipment 30 are shown inFIG. 1, the number of macro-cell base stations may be smaller than twoor larger than four. Furthermore, although some cases, in which thesmall-cell base station 20 performs processes and controls discussedbelow, are described in the following embodiments, any other basestation such as the macro-cell base station 10 may perform similarprocesses and controls. Common sections between the three macro-cellbase station are described with respect to the macro-cell base station10.

The macro-cell base station 10 is a normal wide-area base stationinstalled outdoor in a mobile communication network, which covers amacro cell being a wide area with a radius in a range between about fewhundred meters and about several kilometers, and is sometimes called asa “macro-cell base station”, “Macro e-Node B”, “MeNB” or the like. Themacro-cell base station 10 is connected with other base stations via,for example, a wired communication line, and is configured to be capableof communicating by a predetermined communication interface. Themacro-cell base station 10 is also connected with a core network of themobile communication network via a line-termination device and a leasedline and is configured to be capable of communicating with various typesof nodes in the mobile communication network by a predeterminedcommunication interface.

The small-cell base station 20, which is different from a wide-areamacro-cell base station, is a transferable base station capable of beinginstalled indoor such as in a private house, shop, office or the like,and has a radio-communication range between about few meters and aboutseveral hundred meters. Since the small-cell base station 20 isinstalled so as to covering a area smaller than a area covered with awide-area base station in the mobile communication network, thesmall-cell base station 20 is sometimes called as a “femto base station”or sometimes called as a “Home e-Node B” or “Home eNB”. The small-cellbase station 20 is also connected with a core network of the mobilecommunication network via a line-termination device and a broad-bandpublic communication line such as an ADSL (Asymmetric Digital SubscriberLine), an optical communication line or the like, and is configured tobe capable of communicating with various types of nodes in the mobilecommunication network by a predetermined communication interface.

When being located in the macro cell 10A or the small cell 20A, the userequipment (UE) 30 being as a mobile station used by a user is capable ofperforming a radio communication with the macro-cell base station 10 orthe small-cell base station 20 which corresponds to the serving cell byusing a predetermined communication method and a resource.

FIG. 2A is a function block diagram showing one example of schematicconfiguration of a main part of the user equipment 30 capable ofcommunicating in the mobile communication system according to thepresent embodiment. FIG. 2B is a function block diagram showing oneexample of schematic configuration of a main part of a base stationapparatus 200 forming a small-cell base station 20 according to thepresent embodiment. It is noted that, since a base station apparatus ofthe macro-cell base station 10 located around the small-cell basestation 20 can be configured as same as the small-cell base station 20,a description of the base station apparatus of the macro-cell basestation 10 will be omitted.

The user equipment 30 is configured with, for example, hardware such asa computer device having a CPU, memories, etc., an externalcommunication interface section for the core network, a radiocommunication section and so on, and is capable of performing a radiocommunication with the base station 10, 20, etc. and the like, byexecuting a predetermined program. The base station apparatus 200 isconfigured with, for example, hardware such as a computer device havinga CPU, memories, etc., an external communication interface section forthe core network, a radio communication section and so on, and iscapable of performing various kinds of processes and controls describedbelow such as a memory and update of a list of peripheral base stationsneighboring with the small-cell base station 20, a control of downlinktransmission power, a measurement of an interference level from aperipheral cell, an interference-processing process and so on, andperforming a radio communication with the user equipment 30, byexecuting a predetermined program.

In FIG. 2A, the user equipment 30 is provided with a control section301, a transmission and reception sharing device (DUP: Duplexer) 302, aradio receiving section 303, an OFDM (Orthogonal Frequency DivisionMultiplexing) demodulation section 304, a receiving-quality measurementsection 305 and a notification-information extraction section 306. Theuser equipment 30 is also provided with a P-CQI (Periodic-ChannelQuality Indicator) generation section 307, an SC-FDMA (Single-CarrierFrequency-Division Multiple Access) modulation section 308 and a radiotransmitting section 309.

The control section 301, which, for example, is configured with acomputer device, controls each section based on notification informationextracted by the notification-information extraction section 306, andfunctions as means of passing information on a downlink-signal receivingquality received by the receiving-quality measurement section 305 to theP-CQI generation section 307.

The radio receiving section 303 receives a radio signal modulated with aOFDM system for downlink specified by the LTE from the base stations 10and 20 via an antenna and the transmission and reception sharing device302.

The OFDM demodulation section 304 acquires a reception signal bydemodulating the radio signal modulated with the OFDM system.

The receiving-quality measurement section 305 measures a downlinkreceiving quality (for example, electric field intensity, receivinglevel, etc.) when receiving the downlink radio signal, from thedemodulated by the OFDM demodulation section 304, and passes information(CQI: Channel Quality Indicator) on the measured downlink receivingquality to the control section 301.

The notification-information extraction section 306 extractsnotification information (for example, cell-identification informationsuch as a CGI, a cell ID or the like, location-registration areainformation such as a TAC or the like, control channel information,network version information, etc.) transmitted by the base stations 10and 20, from the reception signal demodulated by the OFDM demodulationsection 304, and passes the extracted notification information to thecontrol section 301.

The P-CQI generation section 307 generates a P-CQI signal being as ameasurement report (Measurement Report) that is periodically transmittedfrom the user equipment 30, based on the information on downlinkreceiving quality (CQI) and the notification information received fromthe control section 301.

The SC-FDMA modulation section 308 modulates various kinds of basebandtransmission signals by using a SC-FDMA (Single-CarrierFrequency-Division Multiple Access) system for uplink specified in theLTE. Particularly, in the present example, by the SC-FDMA modulationsection 308, the transmission signal of P-CQI generated by the P-CQIgeneration section 307 is modulated with the SC-FDMA system.

The radio transmitting section 309 transmits the transmission signalsuch as the P-CQI modulated by the SC-FDMA modulation section 308 to thebase stations 10 and 20 via the transmission and reception sharingdevice 302 and the antenna.

In the description herein, the foregoing P-CQI is a transmission signalincluding the downlink receiving quality information (CQI) and thecell-identification information such as the CGI, cell ID, etc. which areperiodically notified to the base stations 10 and 20 by the userequipment 30. The user equipment 30 may also periodically transmit areference signal (SRS) used for measurements of uplink receiving qualityat the base stations 10 and 20, in addition to the P-CQI. As a physicalchannel for transmitting the P-CQI, for example, a PUCCH (Uplink ControlChannel) format 2 specified in the LTE is used. Radio resources (time,frequency) used for transmission of the P-CQI and SRS are designated bythe base stations 10 and 20.

In FIG. 2B, the base station apparatus 200 is provided with a radiosignal path switching section 201, a transmission and reception sharingdevice (DUP) 202, a downlink radio receiving section 203, an OFDMdemodulation section 204, a notification-information extraction section205, an uplink signal receiving section 206, an SC-FDMA demodulationsection 207 and a receiving-power measurement section 208. The basestation apparatus 200 is also provided with a control section 209 thatperforms a control of transmitting power and the like, a downlink signalgeneration section 210, an OFDM modulation section 211 and a downlinkradio transmitting section 212. It is noted that the base stationapparatus 200 may include an antenna.

The downlink radio receiving section 203 receives a radio signal, whichis modulated with the OFDM for downlink system specified by the LTE andincludes notification information, from the macro-cell base station 10via the antenna, the radio signal path switching section 201 and thetransmission and reception sharing device 202.

The OFDM demodulation section 204 demodulates the radio signal modulatedwith the OFDM system and acquires a reception signal.

The notification-information extraction section 205 extracts thenotification information (for example, information on SIB 2: SystemInformation Block type 2) transmitted by the macro-cell base station 10,from the reception signal demodulated with the OFDM demodulation section204, and passes the extracted notification information to the controlsection 209.

These sections of the downlink radio receiving section 203, the OFDMdemodulation section 204 and the notification-information extractionsection 205 also function as information acquisition means of acquiringinformation of electric field intensity of a transmission signaltransmitted from the macro-cell base station 10 located at a peripheryof the own base station, and measurement means of measuring aninterference level from the macro cell 10A located at a periphery of theown base station.

The uplink signal receiving section 206 receives an uplink radio signaltransmitted by the user equipment 30 that is communication with the basestation 200, via the radio signal path switching section 201 and thetransmission and reception sharing device 202. This radio signalincludes a noise signal such as white noise generated in the radiosignal path switching section 201 and so on, and a radio signal in thepredetermined radio resource and physical channel that are set fortransmitting the aforementioned P-CQI and SRS. When a user equipment(MUE) communicating with the macro-cell base station 10 locatedneighboring with the small-cell base station 20 exists, the radio signalalso includes an uplink signal transmitted from the user equipment(MUE).

The SC-FDMA demodulation section 207 performs a demodulation process ofSC-FDMA system for the reception signal received by the uplink signalreceiving section 206.

The receiving-power measurement section 208 measures a power of thereception signal in the foregoing predetermined radio resource andphysical channel obtained by the demodulation process in the SC-FDMAdemodulation section 207, in each of a single subframe or a plurality ofsubframes, based on the notification information from the peripheralmacro-cell base station 10 which is extracted by thenotification-information extraction section 205. This receiving-powermeasurement section 208 functions as measurement means of measuring apower in a predetermined frequency band assigned to the signal (P-CQI orSRS) that is periodically transmitted from the foregoing user equipment(MUE) to the macro-cell base station 10 located in a periphery of theown base station.

The control section 209 has a memory such as a RAM, ROM or the like andfunctions as memory means of memorizing the neighboring base stationlist as exemplified in Table 1 and Table 2, which is a list ofneighboring base stations located at a periphery of the own basestation. Furthermore, the control section 209 also functions aslist-update means of adding a new peripheral base station in the list ofneighboring base stations when the peripheral base station is found, anddelete means of deleting at least one peripheral base station registeredat an early timing in the list of neighboring base stations when thenumber of peripheral base stations registered in the list of neighboringbase stations reaches to a predetermined maximum value that is set inadvance.

Moreover, the control section 209 also functions as transmission-powercontrol means of controlling a downlink transmission power in a rangebetween a maximum power (Pcell(Max)) and a minimum power (Pcell(Min))which are set in advance, and/or transmission power control means ofcontrolling a downlink transmission power based on a measurement resultof the interference level from the macro cell 10A.

The downlink signal generation section 210 a down link signal to betransmitted to a user equipment 30 located in the cell 20A of the ownbase station.

The OFDM modulation section 211 modulates the downlink signal generatedby the downlink signal generation section 210 with the OFDM system sothat the downlink signal is transmitted with a transmission powerdetermined by the control section 209.

The downlink radio transmitting section 212 transmits the transmissionsignal modulated by the OFDM modulation section 211 via the transmissionand reception sharing device 202, the radio signal path switchingsection 201 and the antenna.

FIG. 3 is a flowchart showing one example of update processing of aneighboring-cell list in the base station apparatus 200 of thesmall-cell base station 20 according to the present embodiment.

In the small-cell base station 20 of the present embodiment, aneighboring-cell list can be created by at least one method of a methodof searching peripheral base stations which is called as “Sniffer” and amethod using measurement reports which is called as “CGI Report”, “UEMR”and the like. In the method of searching peripheral base stations,information on peripheral base stations (cells) is searched with aparticular frequency (for example, 2.1 GHz) at a timing of activating abase station or in a predetermined first cycle (one hour) and/or secondcycle (one week) which are set in advance, and a new peripheral basestation (cell) is registered in the neighboring-cell list when theperipheral base station (cell) is searched. In the method usingreceiving reports, while not being restricted by a particular frequency,a base station collects information on a peripheral base station (cell)based on global identification information (CGI), which is included in ameasurement report (MR) received from a user equipment 30 located in theown cell when performing a handover to a peripheral cell, and registersthe information on peripheral base station (cell) in theneighboring-cell list. In this neighboring-cell list, a maximum value(for example, 64) of registrable neighboring cells is set.

The example in FIG. 3 is an example when performing an update processingof the neighboring-cell list by using the method of searching peripheralbase stations (Sniffer) by the foregoing second cycle (one week).

In FIG. 3, the small-cell base station 20 starts the update processingof the neighboring-cell list by using the method of searching peripheralbase stations (Sniffer) by the foregoing second cycle (one week),searches a neighboring cell located in a periphery of the own cell anddetermines whether the neighboring-cell list is full or not (whetherthere is a free space available for an additional registration in theneighboring-cell list) when acquiring information on a new neighboringcell.

Herein, if the neighboring-cell list is not full, the foregoing acquiredinformation on new neighboring cell is added in the neighboring-celllist.

On the other hand, if the neighboring-cell list is full, information onat least one neighboring cell (peripheral base station) registered at anearly timing in the neighboring-cell list is deleted. For example,information on at least one neighboring cell (peripheral base station)is deleted in the ascending order of the number of measurement reportsfrom the user equipment among the neighboring cells in theneighboring-cell list. Then, the foregoing acquired information on newneighboring cell is added in the neighboring-cell list.

As described above, according to the example in FIG. 3, even in statethat the maximum value of registrable neighboring cells (peripheral basestations) is set in the neighboring-cell list, the information of newneighboring cell (peripheral base station) can be surely added whenperforming the update processing of neighboring-cell list by using theforegoing method of searching peripheral base stations (Sniffer) by thesecond cycle (one week).

FIG. 4 is a flowchart showing another example of update processing ofthe neighboring-cell list in the base station apparatus 200 of thesmall-cell base station 20 according to the present embodiment. Theexample in FIG. 4 is an example when performing an update processing ofthe neighboring-cell list by the foregoing method using measurementreports when performing a handover. In the example in FIG. 4, as an itemin the neighboring-cell list, an item for time information ofcell-identification information and time information such as acquisitiontime of information on a peripheral base station with respect to eachneighboring cell (peripheral base station) is included.

In FIG. 4, the small-cell base station 20 starts to operate a handoverprocessing of a user equipment 30 located in the own cell, then collectsinformation on a neighboring cell (peripheral base station) based oncell-identification information (CGI) that is included in a measurementreport (MR) received from the user equipment 30 located in the own cell,acquires information on the neighboring cell (peripheral base station)and determines whether the neighboring cell (peripheral base station)corresponding to the acquired information is present in theneighboring-cell list or not. Herein, if the neighboring cell(peripheral base station) is present in the neighboring-cell list, timeinformation (time of CGI reporting) of the cell-identificationinformation (CGI) is updated with respect to the neighboring cell.

Next, the small-cell base station 20 determines whether theneighboring-cell list is full or not (whether there is a free spaceavailable for an additional registration in the neighboring-cell list).

Herein, if the neighboring-cell list is not full, the foregoing acquiredinformation on new neighboring cell is added in the neighboring-celllist.

On the other hand, if the neighboring-cell list is full, information onat least one neighboring cell (peripheral base station) registered at anearly timing in the neighboring-cell list is deleted. For example,information on at least one neighboring cell (peripheral base station)is deleted in the ascending order of the time information (time of CGIreporting) of the cell-identification information (CGI) among theneighboring cells in the neighboring-cell list. Then, the foregoingacquired information on new neighboring cell is added in theneighboring-cell list.

As described above, according to the example in FIG. 4, even in statethat the maximum value of registrable neighboring cells (peripheral basestations) is set in the neighboring-cell list, the information of newneighboring cell (peripheral base station) can be surely added whenperforming the update processing of neighboring-cell list by using theforegoing method using measurement reports when performing a handover.

It is noted that, in the examples in FIGS. 3 and 4, when deleting theinformation on neighboring cell (peripheral base station) from theneighboring-cell list, one neighboring cell may be deleted or two ormore neighboring (peripheral base stations) cells may be simultaneouslydeleted.

Furthermore, although a case that the base station apparatus 200installed in the small-cell base station 20 performs the updateprocessing in each of the examples in FIGS. 3 and 4, the same updateprocessing of the neighboring-cell list is applied to a base stationapparatus installed in a macro-cell base station.

FIGS. 5A-5C are respectively a graph showing an example of algorithm ofdownlink transmission power controls (DPCs) different from each other,which are capable of selectively performing when interference from asmall cell 20A to a macro cell 10A is large, in the small-cell basestation 20 according to the present embodiment. The horizontal axis inFIG. 5 shows an interference level (CRS_Ec) of downlink specificreference signal (CRS) from a peripheral macro cell 10A, and thevertical axis shows a downlink transmission power (Pout) [dB] of a basestation apparatus 200 of a small-cell base station 20. Pcell(Max) andPcell(Min) in the figures are respectively the maximum power and theminimum power that can be set in a downlink transmission power control(DPC) in the base station apparatus 200 of the small-cell base station20.

Each algorithm of downlink transmission power controls (DPCs) in FIGS.5A-5C is same as each other until the interference level (CRS_Ec) ofdownlink specific reference signal (CRS) becomes a predeterminedinterference level A [dB] from 0 [dB] in the figures, but is differentfrom each other with respect to a change of downlink transmission powerwhen the interference level (CRS_Ec) becomes the interference level A.

In the algorithm of downlink transmission power control (DPC) in FIG.5A, when the interference level (CRS_Ec) of downlink specific referencesignal (CRS) becomes the predetermined interference level A, thedownlink transmission power Pout is changed to the minimum powerPcell(Min).

In the algorithm of downlink transmission power control (DPC) in FIG.5B, when the interference level (CRS_Ec) of downlink specific referencesignal (CRS) becomes the predetermined interference level A, thedownlink transmission power Pout is changed to 0 [W] so that thedownlink transmission signal is not transmitted.

In the algorithm of downlink transmission power control (DPC) in FIG.5C, when the interference level (CRS_Ec) of downlink specific referencesignal (CRS) becomes the predetermined interference level A, the controlis changed to a CRS enhancement (CRS Booting) control described below,by which a power of CRS is enhanced and a power of signal in otherphysical channel is reduced while maintaining a total power of downlinktransmission signals at the maximum power Pcell(Max).

FIG. 6 is a flowchart showing one example of a downlink transmissionpower control (DPC) in the base station apparatus 200 of small-cell basestation 20 according to the present embodiment.

In FIG. 6, the small-cell base station 20 acquires information onelectric field intensity (hereinafter referred to as “macro-cellelectric field”) of a transmission signal transmitted from a macro-cellbase station located in a periphery of the own base station, by ameasurement or the like, and then determines whether the macro-cellelectric field exceeds a preset macro-cell electric-field upperthreshold (hereinafter referred to as “electric-field upper threshold”)or not.

If the macro-cell electric field exceeds the electric-field upperthreshold, the foregoing acquisition of information on the macro-cellelectric field and the foregoing determination are repeatedly performeda predetermined number of times within a predetermined time. Then, thenumber of times of the macro-cell electric field exceeding theelectric-field upper threshold among the determinations of N times isdefined as “Count”, it is determined whether a value of Count/N exceedsa preset threshold or not.

When the foregoing value of Count/N exceeds the preset threshold, thealgorithm of downlink transmission power control (DPC) is changed to anyone kind of algorithm of downlink transmission power control (DPC) inFIGS. 5A-5C from normal algorithm (algorithm without a CRS enhancement(CRS Booting) in FIG. 5C).

As described above, according to the example in FIG. 6, in state thatthe macro-cell base station is located neighboring the small cell 20A ofsmall-cell base station 20, the interference from the small-cell 20A inthe downlink signals of macro cell 10A can be suppressed withoutlowering the power of downlink signals from the small-cell base station20 more than necessary.

Although it is determined whether the macro-cell electric field exceedsthe electric-field upper threshold or not, instead of this determinationin the example in FIG. 6, it may be determined whether the macro-cellelectric field is equal to or larger than the electric-field upperthreshold. Moreover, although it is determined whether the value ofCount/N exceeds the threshold or not in the example in FIG. 6, insteadof this determination, it may be determined whether the value of Count/Nis equal to or larger than the threshold.

FIG. 7 is a flowchart showing another example of a downlink transmissionpower control (DPC) in the base station apparatus 200 of small-cell basestation 20 according to the present embodiment.

In FIG. 7, after performing the foregoing change of algorithm ofdownlink transmission power control (DPC) in FIG. 6, the small-cell basestation 20 acquires information on the macro-cell electric field by ameasurement or the like, and then determines whether the macro-cellelectric field exceeds a preset macro-cell electric-field lowerthreshold (hereinafter referred to as “electric-field lower threshold”)or not, that is, whether the macro-cell electric field is smaller thanthe electric-field lower threshold or not.

If the macro-cell electric field exceeds the electric-field lowerthreshold, the foregoing acquisition of information on the macro-cellelectric field and the foregoing determination are repeatedly performeda predetermined number of times within a predetermined time. Then, thenumber of times of the macro-cell electric field exceeding theelectric-field lower threshold among the determinations of N times isdefined as “Count”, it is determined whether a value of Count/N exceedsa preset threshold or not.

When the foregoing value of Count/N exceeds the preset threshold, thealgorithm of downlink transmission power control (DPC) is returned tothe normal algorithm (algorithm without a CRS enhancement (CRS Booting)in aforementioned FIG. 5C) from the aforementioned algorithm of downlinktransmission power control (DPC) in FIG. 5A, 5B or 5C after changing inFIG. 6.

As described above, according to the example in FIG. 7, in state thatthe interference from the small-cell 20A in the downlink signals ofmacro cell 10A becomes small due to some sort of causes such as a radiotransmission environment in the periphery of small-cell base station 20,the power of downlink signals from the small-cell base station 20 can bereturned to the maximum power.

Although it is determined whether the macro-cell electric field exceedsthe electric-field lower threshold or not, instead of this determinationin the example in FIG. 7, it may be determined whether the macro-cellelectric field is equal to or less than the electric-field lowerthreshold. Moreover, although it is determined whether the value ofCount/N exceeds the threshold or not in the example in FIG. 7, insteadof this determination, it may be determined whether the value of Count/Nis equal to or larger than the threshold.

FIG. 8 is a graph showing yet another example of algorithm of downlinktransmission power control (DPC) in the base station apparatus 200 ofsmall-cell base station 20 according to the present embodiment. PointsA, B and C in FIG. 8 show the state of CRS Booting that selectivelyenhances a transmission power of downlink specific reference signal(CRS) in the small cell 20A according to the enhancement of interferenceof the downlink specific reference signal (CRS) from the peripheralmacro cell 10A. FIGS. 9A, 9B and 9C are respectively a graph showing acontrol pattern of power of downlink transmission signals with respectto a frequency at the point A, B and C in FIG. 8.

In FIG. 8, until the interference level (CRS_Ec) of downlink specificreference signal (CRS) from the peripheral macro cell 10A becomes theinterference level [dB] at point A from 0 [dB] in the figure, as same asa conventional downlink transmission power control (DPC), any one of anoffset (P_A) of transmission power of downlink specific reference signal(CRS) and an offset (P_B) of transmission power of signals in otherphysical channel to the standard transmission power level Pn is not set(see FIG. 9A).

When the interference level (CRS_Ec) of downlink specific referencesignal (CRS) from the peripheral macro cell 10A reaches a predeterminedinterference level indicated with the point A in FIG. 8, the foregoingoffsets of P_A and P_B are set so as to enhance the transmission powerof downlink specific reference signal (CRS) and to lower thetransmission power of signals in other physical channel, whilemaintaining the total power of downlink transmission signals at thepredetermined maximum power (see FIG. 9B).

Furthermore, when the interference level (CRS_Ec) of downlink specificreference signal (CRS) from the peripheral macro cell 10A is high asshown in the points B and C, according to the high level, the foregoingoffsets of P_A and P_B are set so as to further enhance the transmissionpower of downlink specific reference signal (CRS) and to further lowerthe transmission power of signals in other physical channel, whilemaintaining the total power of downlink transmission signals at thepredetermined maximum power (see FIGS. 9B and 9C).

FIG. 10 is a flowchart showing yet another example of a downlinktransmission power control (DPC) in the base station apparatus 200 ofsmall-cell base station 20 according to the present embodiment. In FIG.10, “*_tmp” shows a temporary value, “*_limit” shows a predeterminedvalue and “*_final” shows a final value. Moreover, “Pmax” shows amaximum output, “CRS” shows a power of resource element per an output ofdownlink specific reference signal (CRS) being as a reference signal and“P_A” shows an offset of transmission power of the downlink specificreference signal (CRS) to a normal transmission power level Pn. Herein,in the standard specification of 3GPP, it is specified that the numberof values available for the output value of P_A is eight (see Non-PatentLiterature 3). When the power value is high, only the values of {−6 dB,−4.77 dB, −3 dB, −1.77 dB} are available. Other values are not availablebecause the maximum value of total power of downlink transmissionsignals exceeds a predetermined value of allowable maximum power. Thevalue of offset (P_B) of transmission power of signals in other physicalchannels except for the CRS is set to the same value as the P_A as shownin the foregoing FIG. 9.

In FIG. 10, to begin with, a temporary value Pmax_tmp of maximum poweris calculated by using a predetermined calculation expression of thedownlink transmission power control (DPC), based on the value ofinterference level (CRS_Ec) of downlink specific reference signal (CRS)from the peripheral macro cell 10A. Moreover, based on this temporaryvalue Pmax_tmp of maximum power, a temporary value CRS_tmp of CRS iscalculated. Then, it is determined whether the temporary value Pmax_tmpof maximum power is equal to or less than the predetermined valuePmax_limit of maximum power or not.

Herein, if the temporary value Pmax_tmp of maximum power is equal to orless than the predetermined value Pmax_limit of maximum power, thetemporary value Pmax_tmp of maximum power is set to the Pmax_final ofmaximum power, the temporary value CRS_tmp of CRS is set to the finalvalue CRS_final of CRS, and the power offset P_A_final is set to 0 dB,and the process terminates.

On the other hand, if the temporary value Pmax_tmp of maximum power islarger than the predetermined value Pmax_limit of maximum power, theprocess is transferred to a setting of the CRS enhancement (CRS Booting)control.

In the setting of the CRS enhancement (CRS Booting) control, to beginwith, a temporary value P_A_temp of power offset for other physicalchannel TO REDUCE to reduce the power is calculated, and it isdetermined whether the temporary value P_A_temp of power offset issmaller than −6 dB or not.

Herein, if the temporary value P_A_temp of power offset is smaller than−6 dB, the final value P_A_final of power offset is set to −6 dB and thefinal value CRS_final of CRS is calculated, and the Pmax_final ofmaximum power is calculated based on the final value P_A_final of poweroffset and the final value CRS_final of CRS.

On the other hand, if the temporary value P_A_temp of power offset isequal to or larger than −6 dB, a value smaller than the temporary valueP_A tmp of power offset for other physical channel is selected from {−6dB, −4.77 dB, −3 dB, −1.77 dB}, the maximum value among the selectedvalues is to be the final value P_A_final of power offset for otherphysical channel. Then, the temporary value CRS_tmp of CRS is set to thefinal value CRS_final, and the Pmax_final of maximum power is calculatedbased on the final value P_A_final of power offset and the final valueCRS_final of CRS.

As described above, according to the examples in FIGS. 8-10, it iscapable of ensuring a sufficiently large coverage of the small cell 20Awhile reducing a power of downlink signals from the small-cell basestation 20 so as to be equal to or smaller than the allowable maximumpower.

It is noted that the description of embodiments disclosed in the presentspecification is provided so that the present disclosures can beproduced or used by those skilled in the art. Various modifications ofthe present disclosures will be readily apparent to those skilled in theart and general principles defined in the present specification can beapplied to other variations without departing from the spirit and scopeof the present disclosures. Therefore, the present disclosures shouldnot be limited to examples and designs described in the presentspecification and should be recognized to be in the broadest scopecorresponding to principles and novel features disclosed in the presentspecification.

REFERENCE SIGNS LIST

-   10 macro-cell base station (peripheral base station)-   10A macro cell-   20 small-cell base station-   20A small cell-   30 user equipment (mobile station)

The invention claimed is:
 1. A base station apparatus installed in asmall-cell base station of a small cell, the small-cell base stationlocated in a macro cell, the base station apparatus in communicationwith a mobile station in a mobile communication network, the basestation apparatus comprising: transmission-power control means ofcontrolling a downlink transmission power of a downlink transmissionsignal in a range between a maximum power and a minimum power which areset in advance; and information acquisition means of acquiringinformation on electrical field intensity of a transmission signaltransmitted from a macro-cell base station located at a periphery of thesmall-cell base station, wherein, in a state that the downlinktransmission power is set to the maximum power and a function ofsuppressing interference between the small cell and the macro cell isnot operated, the transmission-power control means changes the downlinktransmission power so as to change the downlink transmission power fromthe maximum power to the minimum power or stop the transmission of thedownlink transmission signal when the electric field intensity oftransmission signal from the macro-cell base station exceeds a presetelectric-field upper threshold or the electric field intensity is equalto or larger than the electric-field upper threshold, and wherein, in astate that the downlink transmission power is set to the maximum powerand the function of suppressing interference between the small cell andthe macro cell is operated, the transmission-power control meanscontrols so that the maximum power is maintained while not performingthe change of downlink transmission power to change the downlinktransmission power from the maximum power to the minimum power or thestop of the transmission of the downlink transmission signal, even whenthe electric field intensity of transmission signal from the macro-cellbase station exceeds the electric-field upper threshold, or the electricfield intensity is equal to or larger than the electric-field upperthreshold, and wherein the transmission-power control means changes thedownlink transmission power so as to return the downlink transmissionpower to the maximum power, when the electric field intensity ofdownlink signal from the macro-cell base station is smaller than apreset electric-field lower threshold.
 2. The base station apparatusaccording to claim 1, wherein the transmission-power control meanschanges the downlink transmission power so as to return the downlinktransmission power to the maximum power, when the electric fieldintensity is equal to or smaller than the electric-field lowerthreshold, in a state that the downlink transmission power is set to theminimum power or the transmission of downlink transmission signal isstopped.
 3. The base station apparatus according to claim 2, wherein theelectric-field upper threshold and the electric-field lower thresholdare different from each other.
 4. The base station apparatus accordingto claim 2, wherein the electric-field upper threshold and theelectric-field lower threshold are same as each other.
 5. The basestation apparatus according to claim 1, wherein the transmission-powercontrol means performs the change of downlink transmission power afterconfirming that a mobile station is not present in the small cell. 6.The base station apparatus according to claim 1, wherein a number oftimes of determining whether the electric field intensity oftransmission signal from the macro-cell base station exceeds theelectric-field upper threshold, or whether the electric field intensityis equal to or larger than the electric-field upper threshold within apredetermined time is defined as “N times”, wherein the number of timesof the macro-cell electric field exceeding the electric-field upperthreshold or being equal to or larger than the electric-field upperthreshold among the determinations of N times is defined as “Count”, andwherein the change of downlink transmission power is performed to changethe downlink transmission power to the minimum power or stop thetransmission of downlink transmission signal when a value of Countdivided by N exceeds a preset threshold or the value of Count divided byN is equal to or larger than the threshold.
 7. The base stationapparatus according to claim 1, wherein a number of times of determiningwhether the electric field intensity of transmission signal from themacro-cell base station is smaller than the electric-field lowerthreshold, or whether the electric field intensity is equal to orsmaller than the electric-field lower threshold within a predeterminedtime is defined as “N times”, wherein and the number of times of themacro-cell electric field being smaller than the electric-field lowerthreshold or being equal to or smaller than the electric-field lowerthreshold among the determinations of N times is defined as “Count”, andwherein the change of downlink transmission power is performed to returnthe downlink transmission power to the maximum power when a value ofCount divided by N exceeds a preset threshold or the value of Countdivided by N is equal to or larger than the threshold.
 8. A method forsmall-cell base station located in a macro cell, the small-cell basestation having a small cell and in communication with a mobile stationin a mobile communication network, the method comprising: acquiringinformation on electrical field intensity of a transmission signaltransmitted from a macro-cell base station located at a periphery of thesmall-cell base station; and controlling a downlink transmission powerof a downlink transmission signal in a range between a maximum power anda minimum power which are set in advance, including: in a state that thedownlink transmission power is set to the maximum power and a functionof suppressing interference between the small cell and the macro-cell isnot operated, changing the downlink transmission power from the maximumpower to the minimum power or stopping the transmission of the downlinktransmission signal when the electric field intensity of thetransmission signal from the macro-cell base station exceeds a presetelectric-field upper threshold, or the electric field intensity is equalto or larger than the electric-field upper threshold; in a state thatthe downlink transmission power is set to the maximum power and thefunction of suppressing interference between the small cell and themacro cell is operated, maintaining the downlink transmission power atthe maximum power regardless of whether the electric field intensity oftransmission signal from the macro-cell base station exceeds theelectric-field upper threshold, or the electric field intensity is equalto or larger than the electric-field upper threshold; and changing thedownlink transmission power so as to return the downlink transmissionpower to the maximum power, when the electric field intensity ofdownlink signal from the macro-cell base station is smaller than apreset electric-field lower threshold.
 9. The method according to claim8, further comprising: in a state that the downlink transmission poweris set to the minimum power or the transmission of downlink transmissionsignal is stopped, changing the downlink transmission power to returnthe downlink transmission power to the maximum power when the electricfield intensity is equal to or smaller than the electric-field lowerthreshold.
 10. The method according to claim 8, wherein theelectric-field upper threshold and the electric-field lower thresholdhave a different value.
 11. The method according to claim 10, whereinthe electric-field upper threshold and the electric-field lowerthreshold have a same value.
 12. The method according to claim 8,wherein the changing of the downlink transmission power is performedafter confirming that a mobile station is not present in a small cell.13. The method according to claim 8, further comprising: determining anumber of times, N, within a predetermined time that either the electricfield intensity of transmission signal from the macro-cell base stationexceeds the electric-field upper threshold or the electric fieldintensity is equal to or larger than the electric-field upper threshold;and determining a number of times, Count, that the macro-cell electricfield either exceeds the electric-field upper threshold or is equal toor larger than the electric-field upper threshold among thedeterminations of N times, wherein changing the downlink transmissionpower to the minimum power or stopping the transmission of downlinktransmission signal is performed either when a value of Count divided byN exceeds a preset threshold or the value of Count divided by N is equalto or larger than the threshold.
 14. The method according to claim 8,further comprising: determining a number of times, N, within apredetermined time that either the electric field intensity oftransmission signal from the macro-cell base station is smaller than theelectric-field lower threshold or the electric field intensity is equalto or smaller than the electric-field lower threshold; and determining anumber of times, Count, that the macro-cell electric field either issmaller than the electric-field lower threshold or is equal to orsmaller than the electric-field lower threshold among the determinationsof N times, and wherein changing the downlink transmission power isperformed to return the downlink transmission power to the maximum powereither when a value of Count divided by N exceeds a preset threshold orthe value of Count divided by N is equal to or larger than thethreshold.
 15. A system for a small-cell base station located in a macrocell and in communication with a mobile station in a mobilecommunication network, the small-cell base station having a small celland acquiring information on electrical field intensity of atransmission signal transmitted from a macro-cell base station locatedat a periphery of the small-cell base station, the system comprising: acontrol module to control a downlink transmission power of a downlinktransmission signal in a range between a maximum power and a minimumpower which are set in advance, wherein, in a state that the downlinktransmission power is set to the maximum power and a function ofsuppressing interference between the small cell and the macro-cell isnot operated, the control module controls changing the downlinktransmission power from the maximum power to the minimum power orstopping the transmission of the downlink transmission signal when theelectric field intensity of the transmission signal from the macro-cellbase station exceeds a preset electric-field upper threshold, or theelectric field intensity is equal to or larger than the electric-fieldupper threshold, wherein, in a state that the downlink transmissionpower is set to the maximum power and the function of suppressinginterference between the small cell and the macro cell is operated, thecontrol module controls maintaining the downlink transmission power atthe maximum power regardless of whether the electric field intensity oftransmission signal from the macro-cell base station exceeds theelectric-field upper threshold, or the electric field intensity is equalto or larger than the electric-field upper threshold, and wherein, in astate that the downlink transmission power is set to the minimum poweror the transmission of downlink transmission signal is stopped, thecontrol module controls changing the downlink transmission power toreturn the downlink transmission power to the maximum power when theelectric field intensity of the downlink signal from the macro-cell basestation is smaller than a preset electric-field lower threshold.
 16. Thesystem according to claim 15, wherein, in a state that the downlinktransmission power is set to the minimum power or the transmission ofdownlink transmission signal is stopped, the control module controlschanging the downlink transmission power to return the downlinktransmission power to the maximum power when the electric fieldintensity is equal to or smaller than the electric-field lowerthreshold.
 17. The system according to claim 16, wherein the controlmodule controls changing the downlink transmission power afterconfirming that a mobile station is not present in a small cell.
 18. Thesystem according to claim 15, wherein the control module controlsdetermining a number of times, N, within a predetermined time thateither the electric field intensity of transmission signal from themacro-cell base station exceeds the electric-field upper threshold orthe electric field intensity is equal to or larger than theelectric-field upper threshold, wherein the control module controlsdetermining a number of times, Count, that the macro-cell electric fieldeither exceeds the electric-field upper threshold or is equal to orlarger than the electric-field upper threshold among the determinationsof N times, and wherein the control module controls changing thedownlink transmission power to the minimum power or stopping thetransmission of downlink transmission signal is performed either when avalue of Count divided by N exceeds a preset threshold or the value ofCount divided by N is equal to or larger than the threshold.
 19. Thesystem according to claim 15, wherein the control module controlsdetermining a number of times, N, within a predetermined time thateither the electric field intensity of transmission signal from themacro-cell base station is smaller than the electric-field lowerthreshold or the electric field intensity is equal to or smaller thanthe electric-field lower threshold, wherein the control module controlsdetermining a number of times, Count, that the macro-cell electric fieldeither is smaller than the electric-field lower threshold or is equal toor smaller than the electric-field lower threshold among thedeterminations of N times, and wherein the control module controlschanging the downlink transmission power is performed to return thedownlink transmission power to the maximum power either when a value ofCount divided by N exceeds a preset threshold or the value of Countdivided by N is equal to or larger than the threshold.